1. Technical Field
This invention relates to a thread protector for protecting male or female threads formed on the outer or inner surface of a threaded end portion of a metal pipe such as a steel pipe.
2. Description of the Related Art
Steel pipes or more generally metal pipes are typically formed with male or female threads on both ends for connecting pipes with each other in series. Thus, an end portion of a steel pipe is used to form a threaded joint. An end portion of a steel pipe having male threads is called a pin or pin end, while an end portion having female threads is called a box or box end. Some steel pipes have a pin on one end and a box on the other end. Other steel pipes have a pin on both ends and are connected with each other via a coupling having a box or female threads on both sides.
In order to protect pipe threads against rusting and deposition of foreign matter (referred to below simply as dirt) throughout the period of time between the formation of threads and the use of the pipes by a customer and to protect the threads against galling when the threads on two pipes are interconnected, it is common to apply a grease having a relatively high viscosity to the threads. In addition, in order to protect the threads against scratching when pipes rub against each other or against other objects, it is common to fit a thread protector on each end of a pipe so as to cover the threads of the pipe. The thread protector usually has threads for threaded engagement with the pipe, namely, it has female threads for protecting a pin end or male threads for protecting a box end.
A typical conventional thread protector for protecting a pin end of a steel pipe is shown in FIG. 8(a). A metal pipe 2 of steel or other material has male threads 2b on the outer surface of a generally tapered pin formed on an end of the pipe. The end surface 2a of the pipe 2 is designed for abutment against an internal shoulder formed on a box end of another pipe to be connected to pipe 2. The pin end of the pipe 2 is surrounded by a thread protector 1 having female threads 1b to protect the male threads 2b of the pipe 2. The protector 1 has an internal shoulder 1a for abutment against the end surface 2a of the pipe 2, and it is secured to the pipe 2 by being screwed onto the pipe 2 until its shoulder 1a abuts against the end surface 2a of the pipe.
The grease used for protecting threads often contains heavy metals, which pose an environmental hazard. Therefore, in recent years, particularly with OCTG (oil country tubular goods) for use in oil wells, especially in off-shore oil wells, for the production of crude oil, a lubricating method which does not use grease has been proposed, such as in JP-A 9-72467, with the object of preventing environmental pollution.
When such a lubricating method which does not employ grease is used in combination with a conventional thread protector, although it is possible to obtain good lubricating properties when connecting pipes with each other and to adequately prevent scratching of the threads of a pipe, this method has inadequate ability to protect the threads against deposition of dirt and to protect against rusting, which are fractions performed by conventional grease.
Therefore, when using a lubricating method which does not employ grease, it is desired for a thread protector, which has been used in the past to prevent scratching of threads, to perform the additional functions of protecting the against deposition of dirt and rusting. In recent years, there have been various proposals of means for accomplishing these functions in a thread protector.
For example, Japanese Patent No. 3,149,746 discloses a male thread (or pin) protector which increases the ability of the protector to fit on a pin by making the length of the female threads of the protector shorter than the length of the male threads of the pin being protected. However, that thread protector is only suited for use on the male threads of a pipe, and it cannot be used to protect female threads (or a box). In addition, no consideration is given to imparting the protector with the ability to protect pipe threads against dirt and rusting.
U.S. Pat. No. 4,796,668 discloses protectors for the male and female threads of pipes which can protect the threads against dirt and water by providing line contact between lips formed on the protector and surfaces of a pipe on which the protector is mounted.
However, that thread protector has the drawback that manufacture of the protector requires a large number of steps, so it has problems with respect to manufacturing efficiency.
In addition, forming a seal between the protector and the end surface of a pipe being protected relies upon compression of the elastic material forming the body of the protector. Therefore, if a small amount of looseness develops between the protector and a pipe on which it is mounted during the period until the pipe is used by a customer (during this time, pipes may rub against each other when they are transferred between different vehicles such as ships, trains, and trucks, and as a result of this rubbing, a protector may slightly loosen with respect to a pipe), the surface pressure acting between the body of the protector and the steel pipe will decrease, and a gap may even develop between the two. As a result, water may seep to the threads of the pipe from the interior of the pipe.
JP-A 09-303672 discloses a protector for protecting the male threads of a steel pipe which includes an elastic sealing ring located on the interior of the protector for preventing water from seeping from the interior of the pipe on which the protector is mounted to the male threads during a hydrostatic pressure test of the pipe.
However, that thread protector is only capable of protecting the male threads (the pin) of a pipe and cannot be used to protect the female threads (the box) thereof. In addition, since the elastic sealing ring is formed separately from the body of the protector, the two must be combined with each other before the protector can be used, which makes the use of the protector bothersome. Furthermore, when the protector is to be discarded or recycled, it may be necessary to separate the sealing ring from the body of the protector, so the costs of discarding or recycling are increased. In addition, that thread protector does not have any provision for preventing water from seeping to the threads from the exterior of a pipe.
Japanese Patent No. 3,223,874 discloses a protector for protecting male threads which can prevent pressurized water from seeping to the threads of a steel pipe from the interior of the pipe during a hydrostatic pressure test. The protector has an end plate which is contacted by the end surface of a pipe. The thickness of the end plate increases towards the radial center of the protector.
However, that thread protector can only be used to protect the male threads of a pipe, and it cannot be used to protect female threads. In addition, the protector relies solely on the elasticity of the material forming the protector to form a seal against the inner surface of a steel pipe. Therefore, when a slight looseness develops between the protector and a steel pipe on which it is installed during the period until the pipe is used by a customer, the surface pressure acting between the protector and the steel pipe decreases and a gap may even develop between them, so water may seep to the threads of the pipe from the interior of the pipe. In addition, this thread protector does not give any consideration to preventing water from seeping from the exterior of a pipe.
JP-A 11-201371 discloses a thread protector which can protect the threads on the end of a steel pipe without the protector itself having any threads formed on the body of the protector. The protector is comprised of an inner foamed resin layer and an outer non-woven fabric layer. Thus, this thread protector is merely designed to protect the threads of a pipe against damage by impact, and it is not intended to protect the pipe against rusting. Therefore, application of a grease is necessary to protect the threads of a pipe against rusting.
JP-A 2001-199469 discloses a thread protector which can prevent the outflow of an antirust composition such as a grease applied to the reads of a pipe by defining the size of the gaps formed between the threads of a pipe and the threads of the protector. Thus, rust prevention of the threads of a pipe is guaranteed by an antirust composition such as a grease applied to the threads. The protector does not have a sealing mechanism for protecting other portions of the end of a pipe besides the threads against dirt and rusting, such as the sealing surfaces of a pipe which form seals when the pipe is connected to another pipe.
FIGS. 7(a) and 7(b) show another technique which is conceivable in order to protect the threads of a pipe (having a pin end with male threads in FIG. 7(a) or a box end with female threads in FIG. 7(b)) against deposition of dirt and water seepage from the exterior of a steel pipe. In the technique shown in these figures, a gap between a thread protector 1 and a steel pipe 2 is sealed by elastic tape 3 such as plastic adhesive tape, However, with such a technique, when steel pipes rub against each other when being transferred between different vehicles, for example, before they are used by a customer, there is the possibility of the elastic tape tearing and of dirt and water reaching the threads of the pipe through the torn portions of the tape. In addition, with this technique, it is necessary to peel off the elastic tape prior to using a pipe, so the operating efficiency of pipe connection is decreased.